This invention relates to modulated infrared sources and more particularly and generically to modulators in which spatial-on-spatial modulation is provided by the rotation of two sets of beam forming optics about the infrared (IR) source.
The modulation of infrared radiation, due to its long wavelength, has not been accomplished easily in the past. Problems in modulation of infrared energy include those of unwanted refraction due to the long wavelengths as well as absorption of the energy by the apparatus utilized in the modulation technique. Absorbed energy is reradiated in a diffused pattern thereby in many instances degrading the modulation. Additional problems center around materials which can withstand the infrared radiation while the same time being sufficiently lightweight and structurally stable enough to withstand cyclic motion normally employed in the production of a modulated beam. It will be appreciated that when IR sources include heated elements, modulation of the energy to the element is ineffective to cause modulation of the radiation from the element due to the long heating and cooling cycles inherent with the IR sources in which elements are heated.
One of the most important applications of modulated infrared sources is in the area of infrared countermeasures. In this application the modulated infrared source is employed to render ineffective heat seeking missiles which home in on the heat generated by engines which propel targets. These engines include internal combustion engines, jet engines, rocket engines or the like.
In general, it is the purpose of the infrared countermeasure device to produce a modulated infrared signal of sufficiently high intensity to blanket or mask the infrared output from the above mentioned engines. The waveform of the modulation is also important in that some heat seeking missiles respond both to carrier and modulation. In order to accomplish IR countermeasure of these missiles, IR pulses followed by dead space must be present at the missile. Other missiles are countermeasured by IR pulses alone. It is therefore important to provide an IR countermeasure system which can simultaneously countermeasure both classes of missiles. This is accomplished in the subject invention by xe2x80x9cspatial-on-spatial modulationxe2x80x9d. The term xe2x80x9cspatial modulationxe2x80x9d refers to the sweeping of an IR beam past a point in space removed from the IR source. The reason that xe2x80x9cspatialxe2x80x9d modulation is used is because in xe2x80x9cspatialxe2x80x9d modulation of an IR source very little energy from the source is lost.
Modulated infrared sources used for countermeasuring heat seeking missiles exist in the prior art which employ IR sources with temporal modulators. In one embodiment temporal modulation involves the so-called xe2x80x9cchopperxe2x80x9d technique, in which apertures spaced from the source are sequentially covered and uncovered in a shutter technique. However, in these sources when the apertures are covered energy radiated from the IR source is either absorbed by the occluding member or reflected back into the source at a non-optimum angle such that this energy is lost. Where energy for the IR source is virtually unlimited such as is the case when fuel is burned for the production of infrared radiation, temporal modulation techniques work well. Moreover, temporal modulation of electrically powered sources works well if sufficient electrical power is available, as is the case with jet powered fighter aircraft. However, where electrical energy is critically limited, it is desirable that as much of the energy from the IR source as possible be utilized in order that the infrared source radiate sufficient energy to blanket the infrared energy from the target""s engine.
Moreover, to provide omnidirectional or near omnidirectional coverage the infrared source must be omnidirectional so as to be able to countermeasure heat seeking missiles coming in from any direction. In the prior art omnidirectional coverage has been obtained by the provision of a large number of apertures about the IR source. Temporal modulation is obtained by the rotation of a cylindrically shaped mask in front of the apertures. While these systems are effective where unlimited power is available the provision of temporal modulation presents a problem of efficiency which can be critical in many applications because radiation from the source may be blanketed or masked by the radiation from the target.
Moreover, due to the limited power available the coupling of 100% of the power from IR source out of the source is so critical that the shape of the projected infrared image becomes exceedingly important. Assuming a line source, it has been found that refractive optics which would ordinarily focus and couple out a great deal of the energy from the IR source suffer from the fact that the radiated image is not linear. If the image is considered to be rectangular, with the use of refractive optics, the long sides of the rectangle are bent inwardly in a concave manner. Thus, maximum intensity appears at the center of the rectangular image, with the energy being somewhat reduced towards the ends of the image. When energy levels are critical the refractive optics may result in a situation where the energy from the infrared source is enough less than that of the energy from the target""s engines such that the infrared source is blanketed or masked by the infrared energy from the target rather than the other way around. The ratio of infrared energy from the source vis-a-vis infrared energy from the target is called the jam-to-signal ratio and this ratio is a measure of the effectiveness of the infrared source as a countermeasures. It will be appreciated that if this ratio is greater than 1, the infrared source can be effective as a countermeasure.
The above problems are solved by the subject invention in which close to 100% of the infrared energy is coupled out into space by xe2x80x9cspatial-on-spatial modulationxe2x80x9d. Because of the spatial on-spatial modulation, both classes of heat seeking missiles may be simultaneously countermeasured. Generically, in the subject invention spatial-on-spatial modulation is accomplished by rotating two coaxially located sets of focusing optics about a stationary infrared source at a differential rpm commensurate with the modulation frequency and waveform desired. To produce dead space the outer modulator is rotated at a speed considerably lower than that of the inner modulator. The optics in the preferred embodiment are reflective optics although refractive optics may be utilized in situations in which the energy degradation caused by the refractive optics is not critical. Each set of reflective optics in one embodiment comprise narrow parabolically shaped reflectors adjacent an omnidirectional infrared source so that close to 100% of the radiation from the source is coupled out of the system. Moreover, in this system more intense beams are formed due to the additional focusing at the outer modulator. The reflective elements used generate highly defined beams with unusually sharp edges. The source is located at the focii of the parabolas which define the reflector cross sections for both of the modulators, and the parabolas of both modulators are rotated about their focii. Multiple reflectors may be provided to provide multiple beams. In one embodiment the IR source is a line source and the reflectors focus the radiation from this source into a number of vertically extending lines which are projected out away from the source. These lines subtend a vertical angle of almost 180xc2x0 to give omnidirectional coverage with sweep of a lie 360xc2x0 in azimuth.
It is therefore an object of this invention to provide an improved modulated infrared source.
It is another object of andis invention to provide a spatial-on-spatial modulated infrared source.
It is a further object of this invention to provide a spatial modulator for an infrared source in which beam intensity is enhanced.
It is a still further object of this invention to provide reflective focusing by two sets of modulators for an infrared source in which the beam formed by the focusing means is extremely sharp and well defined in the infrared region of the electro-magnetic spectrum.
It is a yet still further object of this invention to provide a modulated omnidirectional infrared radiation source isotropic in azimuth and capable of countermeasuring all known types of heat seeking missiles.